Gas separation by moving solids came into existence with the invention of the first continuous adsorption unit invented by F. D. Soddy in 1922 (U.S. Pat. Nos. 1,422,007 and 1,422,008). The renaissance of Soddy's process for the separation of gas mixtures came around 1946, when C. Berg published results on a moving charcoal bed for fractionation of refinery off gases (Berg. C., AIChE Journal, 42, 665 Aug. 25, 1946). The process was named Hypersorption. Hypersorption as a separation process never gained popularity as a sound unit operation to replace conventional operations due to the mechanical problems that persisted in solid handling, mainly the attrition and consequent loss of the by the adsorbents. The hypersorber had distinct zones as in distillation. It had a stripping section and an enriching section above and below the feed point location. It had a two shell and tube heat exchangers; one at the bottom for regenerating the bed by steam heating and another at the top for cooling the hot regenerated adsorbent before the adsorbent is recycled back to the stripping section. Regeneration by heating is known as thermal swing and was employed in the conventional Hypersorber. However in the proposed process the regeneration is done by depressurization\pressure swing.
Separation brought out in manner similar to that of a Hypersorber was not observed and reported in prior works in fixed beds. The separation mechanism and power of Hypersorption has always remained in relative obscurity, and that is why the concept of a Hypersorber was not emulated for gas separation in fixed beds in earlier works. This is possibly because the idea of a unified approach to separation processes has not been conceived, and hence the real concept of Reflux common to all separation processes was not palpable. The concept of reflux on which the proposed process underlies has been discussed extensively in the paper by D. P. Rao—“The Futility of Raffinate Reflux Revisited” (The Canadian Journal of Chemical Engineering, Volume 77, February, 1999). The proposed process can bring about effective fractionation of any gas mixtures (with separation factor even lower than 2) unlike that in the previous works, wherein the fractionation of air alone was claimed and that too with high selectivity adsorbents. In literature for equilibration separation processes the inherent separation factor corresponds to those product compositions which will be obtained when simple equilibrium is attained between the product phases.
A conventional Pressure swing adsorption (CPSA) unit is employed for separation of binary and multicomponent gas mixtures. A CPSA unit basically involves cycles during which few beds are getting loaded with some component of the gas mixture and few beds that are getting regenerated. The cycle is complete when the said activities in the corresponding bed are complete, and the activities that are opposite to that which took place in the earlier cycle are initiated. This repeated loading and regeneration in each bed will end up in a cyclic steady state over a period of cycles and a separation is achieved. When the separation factor is very large (about 10) sharp separation is possible. However, if the separation factor is between 1.5 and 10, sharp separation cannot be achieved. We get one component to be nearly pure but the recovery is poor. For example, separation of propylene (50 mol %) and propane (50 mol %) by PSA yields about 95% propylene, but the yield is low as low as 50%, which means the rest of propylene is lost with propane. Da Silva and Rodrigues carried out a VSA process with 13×Zeolite that could generate a propylene enriched stream of 98% mol relative to propylene/propane mixture, with 3.2% of nitrogen, a recovery of 19%, and productivity of 0.785 mol/kg/h (Propylene/Propane Separation by Vacuum Swing Adsorption using 13×Zeolite, AIChE Journal, February 2001 Vo. 47, No. 2). However, the said scheme of fractionation with fixed bed hypersorber like process yields sharp separation; i.e. we get nearly pure propylene and propane. The yield is better than 98%.
Air fractionation in fixed beds has been reported in the patent by Sircar et al. (U.S. Pat. No. 4,013,429) with four steps carried out in two trains (2 beds per train). The four steps that have been reported are—Adsorption, Nitrogen Rinse, Desorption and Presaturation. A patent by Knaebel. K. S (U.S. Pat. No. 5,032,150) claims a six step PSA process for the production of high purity nitrogen, and a product oxygen rich gas from air using a fixed bed. The steps are—Blowdown, Purge, Pressurization, Feed, Pressure Equalization and Rinse.
Fractionation of liquid mixtures is achieved using an elutant (desorbent). These processes are well known as Sorbex processes. The Sorbex process is the generic name for similar processes that carry the UOP's (Des Plaines, Ill., USA) trade names like Parex, Ebex, Molex and Sarex. Morbidelli and his coworkers (see Baciocchi et al. (1996) have demonstrated the fractionation gas mixture using Sorbex process. Ruthven and Ching (1989) presented a state-of-the-art review of the countercurrent and simulated countercurrent adsorbers. In the UOP Sorbex process, a single rotary valve having several ports is used to realize the switching of the ports for the injection or withdrawal of liquid. Several pipes have to be connected to the valve and the bed. The bed needs to be sectionalized. The movement of port is discrete.
Ideally, a continuous countercurrent system should involve a bed of adsorbent moving downward in plug flow and a gas/liquid mixture flowing upward in plug flow through the void space. Unfortunately, due to problems of adsorbent attrition, liquid channeling and non-uniform flow of adsorbent particles; such a system has not been successfully developed. The Inventors have noticed that it is possible to use fluidized beds or moving beds to achieve countercurrency. However, the attendant mechanical complexity involved in handling of solids and the loss of adsorbent due to attrition have inhibited the use of the continuous countercurrent processes. These problems are responsible for the demise of the hypersorption process. As an alternative to the moving bed, UOP developed a simulated periodic moving-bed adsorber with a fixed bed. The input and output ports of the adsorber are periodically switched among sections of the bed to achieve countercurrent contact of solid and liquid phases using a rotary valve (U.S. Pat. No. 3,192,954).
Of late sorption enhanced adsorptive reactors have come to limelight as a good prospect for many industrially important reactions that are limited by equilibrium. Hydrogen synthesis reaction by steam reforming of natural gas and ammonia synthesis reaction from synthesis gas (effluent gas obtained by cracking naphtha or natural gas) are two such reactions whose industrial importance has been widely accepted. Rodrigues et al. (Simulation of Five-Step One-Bed Sorption Enhanced Reaction Process, AIChE Journal, December 2002, Vol. 48, No. 12) discusses in detail the modeling and simulation of the steam reforming of natural gas to synthesize hydrogen based on sorption enhanced reaction process. However industrial reactors are normally followed by a huge separation train of varying configurations.
It is felt that fixed bed hypersorber can be suitable for such processes once a proper countercurrent solid-gas/solid-liquid contacting method is devised. Also, such fixed bed hypersorbers can stage process intensification of housing both the reaction as well as separation within a single bed. Thus there is a grave need to develop novel systems for injecting and withdrawing fluids in fixed bed hypersorbers which can effectuate countercurrent solid-gas/solid-liquid contacting.